Two-photon absorption (TPA) occurs through the simultaneous absorption of two or more photons via virtual states in an absorbing medium. For a given chromophore, TPA processes take place at wavelengths much longer than the cut-off wavelength of its linear (single-photon) absorption. In the case of TPA, two quanta of photons may be absorbed from a single light source (degenerate TPA) or two sources of different wavelengths (non-degenerate TPA).
While multiphoton absorption processes have been known since 1931, the field remained dormant largely due to the lack of TPA-active materials with sufficiently large cross-sections. In the mid-1990s, several new classes of chromophores exhibiting very large effective TPA cross-section values, which are generally reported in GM=1×10−50 cm4 s photon−1, were reported. In conjunction with the increased availability of ultrafast high-intensity lasers, the renewed interest has not only sparked a flurry of activities in the preparation of novel dye molecules with enhanced TPA cross-section values, but also many previously conceived applications based on TPA process in photonics and biophotonics are now enabled by these new chromophores. It is important to recognize the following features of two-photon materials technology: (a) upconverted emission, whereby an incident light at lower frequency (energy) can be converted to an output light at higher frequency, for instance, IR to UV-Vis up-conversion; (b) deeper penetration of incident light; (c) highly localized excitation allowing precision control of in-situ photochemical events in the absorbing medium, thereby minimizing undesirable activities such as photodegradation or photobleaching; and (d) fluorescence when properly manipulated allows information feedback. It is anticipated that further ingenious utilization of these basic characteristics will lead to new practical applications, in addition to those already under investigation, e.g., fluorescence imaging, data storage, eye and sensor protection, microfabrication of microelectromechanical systems (MEMS), photodynamic therapy, etc.
In recent years, an intense area of applied photonics research is focused on the utilization of TPA materials in ion sensing and assessment. One of the ion-assay techniques involves two-photon excitation fluorescence microscopy (TPEM). In comparison to traditional microscopy based on one-photon, i.e. UV-Vis-based, processes, TPEM in conjunction with TPA chromophores offers the aforementioned advantages such as increased penetration depth, 3D resolution, reduced phototoxicity, and low background fluorescence. Although these advantages of TPA materials as ion sensors are obvious, the selectivity and sensitivity with respect to environmental conditions, for example pH condition, still need to be improved.
Accordingly, there is a need for new TPA compounds, as well as methods of making the same.